patch 9.0.0172: trying to allocate zero bytes

Problem:    Trying to allocate zero bytes.
Solution:   Do not allocate the proptype array when there are none.
            (closes #10867)
diff --git a/src/textprop.c b/src/textprop.c
index d5a53d6..45023e8 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -736,7 +736,7 @@
     int low = 0;
     int high;
 
-    if (ht == NULL)
+    if (ht == NULL || ht->ht_used == 0)
 	return NULL;
 
     // Make the loopup faster by creating an array with pointers to